Magnetic recording medium having characterized magnetic layer and method for manufacturing same

ABSTRACT

The magnetic recording medium has a non-magnetic support and a magnetic layer which is provided on the support and contains ferromagnetic powder and a binder, in which the ferromagnetic powder is ferromagnetic hexagonal ferrite powder, the magnetic layer contains an abrasive, an intensity ratio (Int (110)/Int (114)) of a peak intensity Int (110) of a diffraction peak of (110) plane of a crystal structure of the hexagonal ferrite, determined by performing X-ray diffraction analysis on the magnetic layer by using an In-Plane method, to a peak intensity Int (114) of a diffraction peak of (114) plane of the crystal structure is equal to or higher than 0.5 and equal to or lower than 4.0, and a squareness ratio in a vertical direction is equal to or higher than 0.65 and equal to or lower than 1.00. The present invention also provides a method for manufacturing the magnetic recording medium.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C 119 to Japanese Patent Application No. 2016-182230 filed on Sep. 16, 2016. The above application is hereby expressly incorporated by reference, in its entirety.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a magnetic recording medium and a method for manufacturing the same.

2. Description of the Related Art

Generally, either or both of the recording of signals on a magnetic recording medium and the reproduction of signals are performed by causing a magnetic head (hereinafter, simply described as “head” as well) to contact and slide on a surface of the magnetic recording medium (a surface of a magnetic layer).

In order to continuously or intermittently repeat the reproduction of the signals recorded on the magnetic recording medium, the head is caused to repeatedly slide on the surface of the magnetic layer. For improving the reliability of the magnetic recording medium as a recording medium for data storage, it is desirable to suppress the deterioration of electromagnetic conversion characteristics during the repeated reproduction (hereinafter, simply described as “deterioration of electromagnetic conversion characteristics” as well). This is because a recording medium in which the electromagnetic conversion characteristics thereof hardly deteriorate during the repeated reproduction can keep exhibiting excellent electromagnetic conversion characteristics even if the reproduction is continuously or intermittently repeated.

Examples of causes of the deterioration of electromagnetic conversion characteristics resulting from repeated reproduction include the occurrence of a phenomenon (referred to as “spacing loss”) in which a distance between the surface of the magnetic layer and the head increases. Examples of causes of the spacing loss include a phenomenon in which while reproduction is being repeated and the head is continuously sliding on the surface of the magnetic layer, foreign substances derived from the magnetic recording medium are attached to the head. In the related art, as a countermeasure for the head attachment occurring as above, an abrasive has been added to the magnetic layer such that the surface of the magnetic layer performs a function of removing the head attachment (for example, see JP2005-243162A).

SUMMARY OF THE INVENTION

It is preferable to add an abrasive to the magnetic layer, because then it is possible to suppress the deterioration of the electromagnetic conversion characteristics resulting from the spacing loss that occurs due to the head attachment. Incidentally, in a case where the deterioration of the electromagnetic conversion characteristics can be suppressed to a level that is higher than the level achieved by the addition of an abrasive to the magnetic layer as in the related art, it is possible to further improve the reliability of the magnetic recording medium as a recording medium for data storage.

The present invention is based on the above circumstances, and an object thereof is to provide a magnetic recording medium in which the electromagnetic conversion characteristics thereof hardly deteriorate even if a head repeatedly slides on a surface of a magnetic layer.

An aspect of the present invention is a magnetic recording medium comprising a non-magnetic support and a magnetic layer which is provided on the support and contains ferromagnetic powder and a binder, in which the ferromagnetic powder is ferromagnetic hexagonal ferrite powder, the magnetic layer contains an abrasive, an intensity ratio (Int (110)/Int (114)) of a peak intensity Int (110) of a diffraction peak of (110) plane of a crystal structure of the hexagonal ferrite, determined by performing X-ray diffraction analysis on the magnetic layer by using an In-Plane method, to a peak intensity Int (114) of a diffraction peak of (114) plane of the crystal structure is equal to or higher than 0.5 and equal to or lower than 4.0, and a squareness ratio in a vertical direction is equal to or higher than 0.65 and equal to or lower than 1.00.

In an aspect, the squareness ratio in a vertical direction is equal to or higher than 0.65 and equal to or lower than 0.90.

In an aspect, the intensity ratio (Int (110)/Int (114)) is equal to or higher than 1.0 and equal to or lower than 3.0.

In an aspect, the magnetic recording medium further comprises a non-magnetic layer containing non-magnetic powder and a binder between the non-magnetic support and the magnetic layer.

Another aspect of the present invention is a method for manufacturing the magnetic recording medium, comprising forming the magnetic layer through a step of preparing a composition for forming a magnetic layer, a step of forming a coating layer by coating the non-magnetic support with the prepared composition for forming a magnetic layer directly or through at least another layer, and a step of performing a vertical alignment treatment on the coating layer, in which the step of preparing the composition for forming a magnetic layer includes a first stage of obtaining a dispersion liquid by performing a dispersion treatment on the ferromagnetic hexagonal ferrite powder, the binder, and a solvent in the presence of first dispersion beads, and a second stage of performing a dispersion treatment on the dispersion liquid obtained by the first stage in the presence of second dispersion beads having a bead size and a density smaller than a bead size and a density of the first dispersion beads.

In an aspect, the second stage is performed in the presence of the second dispersion beads in an amount of equal to or greater than 10 times the amount of the ferromagnetic hexagonal ferrite powder based on mass.

In an aspect, the bead size of the second dispersion beads is equal to or less than 1/100 of the bead size of the first dispersion beads.

In an aspect, the bead size of the second dispersion bead is within a range of 80 to 1,000 nm.

In an aspect, the density of the second dispersion beads is equal to or lower than 3.7 g/cm³.

In an aspect, the second dispersion beads are diamond beads.

According to an aspect of the present invention, it is possible to provide a magnetic recording medium in which the electromagnetic conversion characteristics thereof hardly deteriorate even if a head is caused to repeatedly slide on a surface of a magnetic layer.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

An aspect of the present invention relates to a magnetic recording medium having a non-magnetic support and a magnetic layer which is provided on the support and contains ferromagnetic powder and a binder, in which the ferromagnetic powder is ferromagnetic hexagonal ferrite powder, the magnetic layer contains an abrasive, an intensity ratio (Int (110)/Int (114)) of a peak intensity Int (110) of a diffraction peak of (110) plane of a crystal structure of the hexagonal ferrite, determined by performing X-ray diffraction analysis on the magnetic layer by using an In-Plane method, to a peak intensity Int (114) of a diffraction peak of (114) plane of the crystal structure is equal to or higher than 0.5 and equal to or lower than 4.0, and a squareness ratio in a vertical direction is equal to or higher than 0.70 and equal to or lower than 1.00.

Hereinafter, the intensity ratio (Int (110)/Int (114)) will be described as “XRD (X-ray diffraction) intensity ratio” as well.

In the present invention and the present specification, “surface of the magnetic layer” refers to a surface of the magnetic recording medium on the magnetic layer side. Furthermore, in the present invention and the present specification, “ferromagnetic hexagonal ferrite powder” refers to an aggregate of a plurality of ferromagnetic hexagonal ferrite particles. The ferromagnetic hexagonal ferrite particles are ferromagnetic particles having a hexagonal ferrite crystal structure. Hereinafter, the particles constituting the ferromagnetic hexagonal ferrite powder (ferromagnetic hexagonal ferrite particles) will be described as “hexagonal ferrite particles” or simply as “particles” as well. “Aggregate” is not limited to an aspect in which the particles constituting the aggregate directly contact each other, and also includes an aspect in which a binder, an additive, or the like is interposed between the particles. The same points as described above will also be applied to various powders such as non-magnetic powder in the present invention and the present specification.

In the present invention and the present specification, unless otherwise specified, the description relating to a direction and an angle (for example, “vertical”, “orthogonal”, or “parallel”) includes a margin of error accepted in the technical field to which the present invention belongs. For example, the aforementioned margin of error means a range less than a precise angle±10°. The margin of error is preferably within a precise angle±5°, and more preferably within a precise angle±3°.

Regarding the aforementioned magnetic recording medium, the inventor of the present invention made assumptions as below.

The magnetic layer of the magnetic recording medium contains an abrasive. The addition of the abrasive to the magnetic layer enables the surface of the magnetic layer to perform a function of removing head attachment. However, it is considered that in a case where the abrasive present on the surface of the magnetic layer and/or in the vicinity of the surface of the magnetic layer fails to appropriately permeate the inside of the magnetic layer by the force applied thereto from the head when the head is sliding on the surface of the magnetic layer, the head will be scraped by contacting the abrasive protruding from the surface of the magnetic layer (head scraping). It is considered that in a case where the head scraping that occurs as above can be inhibited, it is possible to further suppress the deterioration of the electromagnetic conversion characteristics caused by the spacing loss.

Regarding the aforementioned point, the inventors of the present invention assume that in the ferromagnetic hexagonal ferrite powder contained in the magnetic layer include particles (hereinafter, referred to as “former particles”) which exert an influence on the degree of permeation of the abrasive by supporting the abrasive pushed into the inside of the magnetic layer and particles (hereinafter, referred to as “latter particles”) which are considered not to exert such an influence or to exert such an influence to a small extent. It is considered that the latter particles are fine particles resulting from partial chipping of particles due to the dispersion treatment performed at the time of preparing a composition for forming a magnetic layer, for example. The inventors of the present invention also assume that the more the fine particles contained in the magnetic layer, the further the hardness of the magnetic layer decreases, although the reason is unclear. In a case where the hardness of the magnetic layer decreases, the surface of the magnetic layer is scraped when the head slides on the surface of the magnetic layer (magnetic layer scraping), the foreign substances occurring due to the scraping are interposed between the surface of the magnetic layer and the head, and as a result, spacing loss occurs.

The inventors of the present invention consider that in the ferromagnetic hexagonal ferrite powder present in the magnetic layer, the former particles are particles resulting in a diffraction peak in X-ray diffraction analysis using an In-Plane method, and the latter particles do not result in a diffraction peak or exert a small influence on a diffraction peak because they are fine. Therefore, the inventors of the present invention assume that based on the intensity of the diffraction peak determined by X-ray diffraction analysis performed on the magnetic layer by using the In-Plane method, the way the particles, which support the abrasive pushed into the inside of the magnetic layer and exert an influence on the degree of permeation of the abrasive, are present in the magnetic layer can be controlled, and as a result, the degree of permeation of the abrasive can be controlled. The inventors of the present invention consider that the XRD intensity ratio, which will be specifically described later, is a parameter relating to the aforementioned point.

Meanwhile, the squareness ratio in a vertical direction is a ratio of remnant magnetization to saturation magnetization measured in a direction perpendicular to the surface of the magnetic layer. The smaller the remnant magnetization, the lower the ratio. Presumably, it is difficult for the latter particles to retain magnetization because they are fine. Therefore, presumably, as the amount of the latter particles contained in the magnetic layer increases, the squareness ratio in a vertical direction tends to be reduced. Accordingly, the inventors of the present invention consider that the squareness ratio in a vertical direction can be a parameter of the amount of the latter particles (fine particles) present in the magnetic layer. Presumably, as the amount of such fine particles contained in the magnetic layer increases, the hardness of the magnetic layer may decrease. The inventors of the present invention consider that, accordingly, the surface of the magnetic layer is scraped when the head slides on the surface of the magnetic layer, the foreign substances that occur due to the scraping may be interposed between the surface of the magnetic layer and the head, and hence the spacing loss strongly tends to occur.

In the aforementioned magnetic recording medium, each of the XRD intensity ratio and the squareness ratio in a vertical direction is in the aforementioned range, and hence the deterioration of the electromagnetic conversion characteristics during repeated sliding can be suppressed. According to the inventors of the present invention, presumably, this is because the control of the XRD intensity ratio mainly makes it possible to inhibit the head scraping, and the control of the squareness ratio in a vertical direction mainly makes it possible to inhibit the magnetic layer scraping.

The points described so far are assumptions that the inventors of the present invention made regarding the mechanism which makes it possible to suppress the deterioration of the electromagnetic conversion characteristics in the magnetic recording medium even if sliding is repeated.

However, the present invention is not limited to the assumption.

Hereinafter, the magnetic recording medium will be more specifically described. The present specification includes the assumption made by the inventors of the present invention, but the present invention is not limited to the assumption.

[XRD Intensity Ratio]

In the magnetic recording medium, the magnetic layer contains ferromagnetic hexagonal ferrite powder. The XRD intensity ratio is determined by performing X-ray diffraction analysis on the magnetic layer containing the ferromagnetic hexagonal ferrite powder by using an In-Plane method. Hereinafter, the X-ray diffraction analysis performed using an In-Plane method will be described as “In-Plane XRD” as well. In-Plane XRD is performed by irradiating the surface of the magnetic layer with X-rays by using a thin film X-ray diffractometer under the following conditions. Magnetic recording media are roughly classified into a tape-like magnetic recording medium (magnetic tape) and a disc-like magnetic recording medium (magnetic disc). The magnetic tape is measured in a longitudinal direction, and the magnetic disc is measured in a radius direction.

Radiation source used: Cu radiation (power of 45 kV, 200 mA)

Scan condition: 0.05 degree/step within a range of 20 to 40 degree, 0.1 degree/min

Optical system used: parallel optical system

Measurement method: 2 θχ scan (X-ray incidence angle: 0.25°)

The above conditions are value set in the thin film X-ray diffractometer. As the thin film X-ray diffiactometer, known instruments can be used. As one of the thin film X-ray diffractometers, SmartLab manufactured by Rigaku Corporation can be exemplified. The sample used for In-Plane XRD analysis is not particularly limited in terms of the size and shape, as long as it is a medium sample which is cut from a magnetic recording medium to be measured and enables the confirmation of a diffraction peak which will be described later.

Examples of the techniques of X-ray diffraction analysis include thin film X-ray diffraction and powder X-ray diffraction. By the powder X-ray diffraction, the X-ray diffraction of a powder sample is measured. In contrast, by the thin film X-ray diffraction, it is possible to measure the X-ray diffraction of a layer formed on a substrate and the like. The thin film X-ray diffraction is classified into an In-Plane method and an Out-Of-Plane method. In the Out-Of-Plane method, the X-ray incidence angle during measurement is within a range of 5.00° to 90.00°. In contrast, in the In-Plane method, the X-ray incidence angle is generally within a range of 0.20° to 0.50°. In the present invention and the present specification, the X-ray incidence angle in In-Plane XRD is set to be 0.25° as described above. In the In-Plane method, the X-ray incidence angle is smaller than in the Out-Of-Plane method, and hence the X-ray permeation depth is small. Accordingly, by the X-ray diffraction analysis (In-Plane XRD) using the In-Plane method, it is possible to analyze the X-ray diffraction of a surface layer portion of a sample to be measured. For the sample of the magnetic recording medium, the X-ray diffraction of the magnetic layer can be analyzed by In-Plane XRD. In an X-ray diffraction spectrum obtained by the aforementioned In-Plane XRD, the aforementioned XRD intensity ratio is an intensity ratio (Int (110)/Int (114)) of a peak intensity Int (110) of a diffraction peak of (110) plane of a crystal structure of the hexagonal ferrite to a peak intensity Int (114) of a diffraction peak of (114) plane of the crystal structure. Int is used as the abbreviation of intensity. In the X-ray diffraction spectrum obtained by In-Plane XRD (ordinate: intensity, abscissa: diffraction angle 2 θχ (degree)), the diffraction peak of (114) plane is a peak detected at 2 θχ that is within a range of 33 to 36 degree, and the diffraction peak of (110) plane is a peak detected at 2 θχ that is within a range of 29 to 32 degree.

Among diffraction planes, (114) plane of the crystal structure of the hexagonal ferrite is positioned close to a direction of a magnetization easy axis (c-axis direction) of the particles of the ferromagnetic hexagonal ferrite powder (hexagonal ferrite particles). The (110) plane of the hexagonal ferrite crystal structure is positioned in a direction orthogonal the direction of the magnetization easy axis.

Regarding the aforementioned former particles among the hexagonal ferrite particles contained in the magnetic layer, the inventors of the present invention considered that the more the direction of the particles orthogonal to the magnetization easy axis is parallel to the surface of the magnetic layer, the more difficult it is for the abrasive to permeate the inside of the magnetic layer by being supported by the hexagonal ferrite particles. In contrast, regarding the former particles in the magnetic layer, the inventors of the present invention consider that the more the direction of the particles orthogonal to the magnetization easy axis is perpendicular to the surface of the magnetic layer, the easier it is for the abrasive to permeate the inside of the magnetic layer because it is difficult for the abrasive to be supported by the hexagonal ferrite powder. Furthermore, the inventors of the present invention assume that in the X-ray diffraction spectra determined by In-Plane XRD, in a case where the intensity ratio (Int (110)/Int (114); XRD intensity ratio) of the peak intensity Int (110) of the diffraction peak of (110) plane to the peak intensity Int (114) of the diffraction peak of (114) plane of the hexagonal ferrite crystal structure is high, it means that the magnetic layer contains a large amount of the former particles whose direction orthogonal to the direction of the magnetization easy axis is more parallel to the surface of the magnetic layer; and in a case where the XRD intensity ratio is low, it means that the magnetic layer contains a small amount of such former particles. In addition, the inventors consider that in a case where the XRD intensity ratio is equal to or lower than 4.0, it means that the former particles, that is, the particles, which support the abrasive pushed into the inside of the magnetic layer and exert an influence on the degree of the permeation of the abrasive, merely support the abrasive, and as a result, the abrasive can appropriately permeate the inside of the magnetic layer at the time when a head slides on the surface of the magnetic layer. The inventors of the present invention assume that the aforementioned mechanism may make a contribution to hinder the occurrence of the head scraping even if the head repeatedly slides on the surface of the magnetic layer. In contrast, the inventors of the present invention consider that the state in which the abrasive appropriately protrudes from the surface of the magnetic layer when the head slides on the surface of the magnetic layer may make a contribution to reduce the contact area (real contact) between the surface of the magnetic layer and the head. The inventors consider that the larger the real contact area, the stronger the force applied to the surface of the magnetic layer from the head when the head slides on the surface of the magnetic layer, and as a result, the surface of the magnetic layer is damaged and scraped. Regarding this point, the inventors of the present invention assume that in a case where the XRD intensity ratio is equal to or higher than 0.5, it shows that the aforementioned former particles are present in the magnetic layer in a state of being able to support the abrasive with allowing the abrasive to appropriately protrude from the surface of the magnetic layer when the head slides on the surface of the magnetic layer.

From the viewpoint of further suppressing the deterioration of the electromagnetic conversion characteristics, the XRD intensity ratio is preferably equal to or lower than 3.5, and more preferably equal to or lower than 3.0. From the same viewpoint, the XRD intensity ratio is preferably equal to or higher than 0.7, and more preferably equal to or higher than 1.0. The XRD intensity ratio can be controlled by the treatment conditions of the alignment treatment performed in the manufacturing process of the magnetic recording medium. As the alignment treatment, it is preferable to perform a vertical alignment treatment. The vertical alignment treatment can be preferably performed by applying a magnetic field in a direction perpendicular to a surface of the wet (undried) coating layer of the composition for forming a magnetic layer. The further the alignment conditions are strengthened, the higher the XRD intensity ratio tends to be. Examples of the treatment conditions of the alignment treatment include the magnetic field intensity in the alignment treatment and the like. The treatment conditions of the alignment treatment are not particularly limited, and may be set such that an XRD intensity ratio of equal to or higher than 0.5 and equal to or lower than 4.0 can be achieved. For example, the magnetic field intensity in the vertical alignment treatment can be set to be 0.10 to 0.80 T or 0.10 to 0.60 T. As the dispersibility of the ferromagnetic hexagonal ferrite powder in the composition for forming a magnetic layer is improved, the value of the XRD intensity ratio tends to increase by the vertical alignment treatment.

[Squareness Ratio in Vertical Direction]

The squareness ratio in a vertical direction is a squareness ratio measured in a vertical direction of the magnetic recording medium. “Vertical direction” described regarding the squareness ratio refers to a direction orthogonal to the surface of the magnetic layer. For example, in a case where the magnetic recording medium is a tape-like magnetic recording medium, that is, a magnetic tape, the vertical direction is a direction orthogonal to a longitudinal direction of the magnetic tape. The squareness ratio in a vertical direction is measured using a vibrating sample fluxmeter. Specifically, in the present invention and the present specification, the squareness ratio in a vertical direction is a value determined by carrying out scanning in the vibrating sample fluxmeter by applying a maximum external magnetic field of 1,194 kA/m (15 kOe) as an external magnetic field to the magnetic recording medium, at a measurement temperature of 23° C.±1° C. under the condition of a scan rate of 4.8 kA/m/sec (60 Oe/sec), which is used after being corrected for a demagnetizing field. The measured squareness ratio is a value from which the magnetization of a sample probe of the vibrating sample fluxmeter is subtracted as background noise.

The squareness ratio in a vertical direction of the magnetic recording medium is equal to or higher than 0.65. The inventors of the present invention assume that the squareness ratio in a vertical direction of the magnetic recording medium can be a parameter of the amount of the aforementioned latter particles (fine particles) present in the magnetic layer that are considered to induce the reduction in the hardness of the magnetic layer. It is considered that the magnetic layer in the magnetic recording medium having a squareness ratio in a vertical direction of equal to or higher than 0.65 has high hardness because of containing a small amount of such fine particles and is hardly scraped by the sliding of the head on the surface of the magnetic layer. Presumably, because the surface of the magnetic layer is hardly scraped, it is possible to inhibit the electromagnetic conversion characteristics from deteriorating due to the occurrence of spacing loss resulting from foreign substances that occur due to the scraping of the surface of the magnetic layer. From the viewpoint of further suppressing the deterioration of the electromagnetic conversion characteristics, the squareness ratio in a vertical direction is preferably equal to or higher than 0.70, more preferably equal to or higher than 0.73, and even more preferably equal to or higher than 0.75. In principal, the squareness ratio is 1.00 at most. Accordingly, the squareness ratio in a vertical direction of the magnetic recording medium is equal to or lower than 1.00. The squareness ratio in a vertical direction may be equal to or lower than 0.95, 0.90, 0.87, or 0.85, for example. The larger the value of the squareness ratio in a vertical direction, the smaller the amount of the aforementioned fine latter particles in the magnetic layer. Therefore, it is considered that from the viewpoint of the hardness of the magnetic layer, the value of the squareness ratio is preferably large. Accordingly, the squareness ratio in a vertical direction may be higher than the upper limit exemplified above.

The inventors of the present invention consider that in order to obtain a squareness ratio in a vertical direction of equal to or higher than 0.65, it is preferable to inhibit fine particles from occurring due to partial chipping of particles in the step of preparing the composition for forming a magnetic layer. Specific means for inhibiting the occurrence of chipping will be described later.

Hereinafter, the magnetic recording medium will be more specifically described.

[Magnetic Layer]

<Ferromagnetic Hexagonal Ferrite Powder>

The magnetic layer of the magnetic recording medium contains ferromagnetic hexagonal ferrite powder as ferromagnetic powder. Regarding the ferromagnetic hexagonal ferrite powder, a magnetoplumbite type (referred to as “M type” as well), a W type, a Y type, and Z type are known as crystal structures of the hexagonal ferrite. The ferromagnetic hexagonal ferrite powder contained in the magnetic layer may take any of the above crystal structures. The crystal structures of the hexagonal ferrite contain an iron atom and a divalent metal atom as constituent atoms. The divalent metal atom is a metal atom which can become a divalent cation as an ion, and examples thereof include alkali earth metal atoms such as a barium atom, a strontium atom, and a calcium atom, a lead atom, and the like. For example, the hexagonal ferrite containing a barium atom as a divalent metal atom is barium ferrite, and the hexagonal ferrite containing a strontium atom is strontium ferrite. The hexagonal ferrite may be a mixed crystal of two or more kinds of hexagonal ferrite. As one of the mixed crystals, a mixed crystal of barium ferrite and strontium ferrite can be exemplified.

As the parameter of a particle size of the ferromagnetic hexagonal ferrite powder, activation volume can be used. “Activation volume” is the unit of magnetization inversion. The activation volume described in the present invention and the present specification is a value measured using a vibrating sample fluxmeter in an environment with an atmospheric temperature of 23° C.±1° C. by setting a magnetic field sweep rate to be 3 minutes and 30 minutes for a coercive force Hc measurement portion, and determined from the following relational expression of Hc and an activation volume V. Hc=2Ku/Ms{1−[(kT/KuV)ln(At/0.693)]^(1/2)} [In the expression, Ku: anisotropy constant, Ms: saturation magnetization, k: Boltzmann constant, T: absolute temperature, V: activation volume, A: spin precession frequency, t: magnetic field inversion time]

Recently, as the amount of information has enormously increased, the improvement of recording density of magnetic recording media (high-density recording) has been required. Examples of methods for achieving the high-density recording include a method of increasing a filling rate of ferromagnetic powder in the magnetic layer by reducing the particle size of the ferromagnetic powder contained in the magnetic layer. In this respect, the activation volume of the ferromagnetic hexagonal ferrite powder is preferably equal to or less than 2,500 nm³, more preferably equal to or less than 2,300 nm³, and even more preferably equal to or less than 2,000 nm³. In contrast, from the viewpoint of the stability of magnetization, the activation volume is preferably equal to or greater than 800 nm³, more preferably equal to or greater than 1,000 nm³, and even more preferably equal to or greater than 1,200 nm³, for example. In some cases, the activation volume of the ferromagnetic hexagonal ferrite powder (hereinafter, described as “raw material powder” as well) used for preparing the composition for forming a magnetic layer is the same as or different from the activation volume of the ferromagnetic hexagonal ferrite powder in the magnetic layer formed using the prepared composition for forming a magnetic layer.

In order to identify the shape of the particles constituting the ferromagnetic hexagonal ferrite powder, the ferromagnetic hexagonal ferrite powder is imaged using a transmission electron microscope at a 100,000× magnification, and the image is printed on photographic paper such that the total magnification thereof becomes 500,000×. In the image of the particles obtained in this way, the outlines of particles (primary particles) are traced using a digitizer so as to identify the particle shape. The primary particles refer to independent particles not being aggregated with each other. The particles are imaged using a transmission electron microscope at an acceleration voltage of 300 kV by using a direct method. For performing observation and measurement using the transmission electron microscope, for example, it is possible to use a transmission electron microscope H-9000 manufactured by Hitachi High-Technologies Corporation and image analysis software KS-400 manufactured by Carl Zeiss AG Regarding the shape of the particles constituting the ferromagnetic hexagonal ferrite powder, “plate-like” means a shape having two plate surfaces facing each other. Among particle shapes that do not have such plate surfaces, a shape having a major axis and a minor axis different from each other is “elliptical”. The major axis is an axis (straight light) which is the longest diameter of a particle. The minor axis is a straight line which is the longest diameter of a particle in a direction orthogonal to the major axis. A shape in which the major axis and the minor axis are the same as each other, that is, a shape in which the major axis length equals the minor axis length is “spherical”. A shape in which the major axis and the minor axis cannot be identified is called “amorphous”. The imaging performed for identifying the particle shape by using a transmission electron microscope is carried out without performing an alignment treatment on the powder to be imaged. The raw material powder used for preparing the composition for forming a magnetic layer and the ferromagnetic hexagonal ferrite powder contained in the magnetic layer may take any of the plate-like shape, the elliptical shape, the spherical shape and the amorphous shape.

The mean particle size relating to various powders described in the present invention and the present specification is an arithmetic mean of sizes determined for 500 particles randomly extracted using a particle image captured as described above. The mean particle size shown in examples which will be described later is a value obtained using H-9000 manufactured by Hitachi High-Technologies Corporation as a transmission electron microscope and image analysis software KS-400 manufactured by Carl Zeiss AG as image analysis software.

For details of the ferromagnetic hexagonal ferrite powder, for example, paragraphs “0134” to “0136” in JP2011-216149A can also be referred to.

The content (filling rate) of the ferromagnetic hexagonal ferrite powder in the magnetic layer is preferably within a range of 50% to 90% by mass, and more preferably within a range of 60% to 90% by mass. The magnetic layer contains at least a binder and an abrasive as components other than the ferromagnetic hexagonal ferrite powder, and can optionally contain one or more kinds of additives. From the viewpoint of improving the recording density, the filling rate of the ferromagnetic hexagonal ferrite powder in the magnetic layer is preferably high.

<Binder and Curing Agent>

The magnetic layer of the magnetic recording medium contains a binder. As the binder, one or more kinds of resins are used. The resin may be a homopolymer or a copolymer. As the binder contained in the magnetic layer, a binder selected from an acryl resin obtained by copolymerizing a polyurethane resin, a polyester resin, a polyamide resin, a vinyl chloride resin, styrene, acrylonitrile, or methyl methacrylate, a cellulose resin such as nitrocellulose, an epoxy resin, a phenoxy resin, a polyvinyl alkyral resin such as polyvinyl acetal or polyvinyl butyral can be used singly, or a plurality of resins can be used by being mixed together. Among these, a polyurethane resin, an acryl resin, a cellulose resin, and a vinyl chloride resin are preferable. These resins can be used as a binder in a non-magnetic layer and/or a back coating layer which will be described later. Regarding the aforementioned binders, paragraphs “0029” to “0031” in JP2010-24113A can be referred to. The average molecular weight of the resin used as a binder can be equal to or greater than 10,000 and equal to or less than 200,000 in terms of a weight-average molecular weight, for example. The weight-average molecular weight in the present invention and the present specification is a value determined by measuring a molecular weight by gel permeation chromatography (GPC) and expressing the molecular weight in terms of polystyrene. As the measurement conditions, the following conditions can be exemplified. The weight-average molecular weight shown in examples which will be described later is a value determined by measuring a molecular weight under the following measurement conditions and expressing the molecular weight in terms of polystyrene.

GPC instrument: HLC-8120 (manufactured by Tosoh Corporation)

Column: TSK gel Multipore HXL-M (manufactured by Tosoh Corporation, 7.8 mmID (Inner Diameter)×30.0 cm)

Eluent: tetrahydrofuran (THF)

At the time of forming the magnetic layer, it is possible to use a curing agent together with a resin usable as the aforementioned binder. In an aspect, the curing agent can be a thermosetting compound which is a compound experiencing a curing reaction (cross-linking reaction) by heating. In another aspect, the curing agent can be a photocurable compound experiencing a curing reaction (cross-linking reaction) by light irradiation. The curing agent experiences a curing reaction in the manufacturing process of the magnetic recording medium. In this way, at least a portion of the curing agent can be contained in the magnetic layer, in a state of reacting (cross-linked) with other components such as the binder. The curing agent is preferably a thermosetting compound which is suitably polyisocyanate. For details of polyisocyanate, paragraphs “0124” and “0125” in JP2011-216149A can be referred to. The curing agent can be used by being added to the composition for forming a magnetic layer, in an amount of 0 to 80.0 parts by mass with respect to 100.0 parts by mass of the binder and preferably in an amount of 50.0 to 80.0 parts by mass from the viewpoint of improving the hardness of the magnetic layer.

<Abrasive>

The magnetic layer of the magnetic recording medium contains an abrasive. The abrasive refers to non-magnetic powder having a Mohs hardness of higher than 8, and is preferably non-magnetic powder having a Mohs hardness of equal to or higher than 9. The abrasive may be powder of an inorganic substance (inorganic powder) or powder of an organic substance (organic powder), and is preferably inorganic powder. The abrasive is preferably inorganic powder having a Mohs hardness of higher than 8, and even more preferably inorganic powder having Mohs hardness of equal to or higher than 9. The maximum value of the Mohs hardness is 10 which is the Mohs hardness of diamond. Specific examples of the abrasive include powder of alumina (Al₂O₃), silicon carbide, boron carbide (B₄C), TiC, cerium oxide, zirconium oxide (ZrO₂), diamond, and the like. Among these, alumina powder is preferable. Regarding the alumina powder, paragraph “0021” in JP2013-229090A can also be referred to. As a parameter of the particle size of the abrasive, specific surface area can be used. The larger the specific surface area, the smaller the particle size. It is preferable to use an abrasive having a specific surface area (hereinafter, described as “BET specific surface area”) of equal to or greater than 14 m²/g, which is measured for primary particles by a Brunauer-Emmett-Teller (BET) method. From the viewpoint of dispersibility, it is preferable to use an abrasive having a BET specific surface area of equal to or less than 40 m²/g. The content of the abrasive in the magnetic layer is preferably 1.0 to 20.0 parts by mass with respect to 100.0 parts by mass of the ferromagnetic hexagonal ferrite powder.

<Additive>

The magnetic layer contains the ferromagnetic hexagonal ferrite powder, the binder, and the abrasive, and may further contain one or more kinds of additives if necessary. As one of the additives, the aforementioned curing agent can be exemplified. Examples of the additives that can be contained in the magnetic layer include a non-magnetic filler, a lubricant, a dispersant, a dispersion aid, a fungicide, an antistatic agent, an antioxidant, carbon black, and the like. The non-magnetic filler has the same definition as non-magnetic powder. Examples of the non-magnetic filler include non-magnetic fillers which can make a contribution to the control of frictional characteristics of the surface of the magnetic layer. As such non-magnetic fillers, it is possible to use various non-magnetic powders generally used in the magnetic layer. The powders may be inorganic powder or organic powder. In an aspect, from the viewpoint of uniformizing the frictional characteristics, it is preferable that the particle size distribution of the non-magnetic filler is not polydisperse distribution having a plurality of peaks but monodisperse distribution showing a single peak. From the viewpoint of ease of availability of the monodisperse particles, the non-magnetic filler is preferably inorganic powder. Examples of the inorganic powder include powder of a metal oxide, a metal carbonate, a metal sulfate, a metal nitride, a metal carbide, a metal sulfide, and the like. The particles constituting the non-magnetic filler are preferably colloidal particles, and more preferably colloidal particles of an inorganic oxide. From the viewpoint of ease of availability of the monodisperse particles, the inorganic oxide constituting the colloidal particles of an inorganic oxide is preferably silicon dioxide (silica). The colloidal particles of an inorganic oxide are preferably colloidal silica (colloidal silica particles). In the present invention and the present specification, “colloidal particles” refer to the particles which can form a colloidal dispersion by being dispersed without being precipitated in a case where the particles are added in an amount of 1 g per 100 mL of at least one organic solvent among methyl ethyl ketone, cyclohexanone, toluene, ethyl acetate, and a mixed solvent containing two or more kinds of the solvents described above at any mixing ratio. In another aspect, the non-magnetic filler is also preferably carbon black. The mean particle size of the non-magnetic filler is 30 to 300 nm for example, and preferably 40 to 200 nm. The content of the non-magnetic filler in the magnetic layer is, with respect to 100.0 parts by mass of the ferromagnetic hexagonal ferrite powder, preferably 1.0 to 4.0 parts by mass and more preferably 1.5 to 3.5 parts by mass, because then the filler can demonstrate better the function thereof.

As one of the additives which can be used in the magnetic layer containing the abrasive, the dispersant described in paragraphs “0012” to “0022” in JP2013-131285A can be exemplified as a dispersant for improving the dispersibility of the abrasive in the composition for forming a magnetic layer.

Examples of the dispersant also include known dispersants such as a carboxy group-containing compound and a nitrogen-containing compound. The nitrogen-containing compound may be any one of a primary amine represented by NH₂R, a secondary amine represented by NHR₂, and a tertiary amine represented by NR₃, for example. R represents any structure constituting the nitrogen-containing compound, and a plurality of R's present in the compound may be the same as or different from each other. The nitrogen-containing compound may be a compound (polymer) having a plurality of repeating structures in a molecule. The inventors of the present invention consider that because the nitrogen-containing portion of the nitrogen-containing compound functions as a portion adsorbed onto the surface of particles of the ferromagnetic powder, the nitrogen-containing compound can act as a dispersant. Examples of the carboxy group-containing compound include fatty acids such as oleic acid. Regarding the carboxy group-containing compound, the inventors of the present invention consider that because the carboxy group functions as a portion adsorbed onto the surface of particles of the ferromagnetic powder, the carboxy group-containing compound can act as a dispersant. It is also preferable to use the carboxy group-containing compound and the nitrogen-containing compound in combination.

The additives can be used by being appropriately selected from commercially available products or those manufactured by known methods according to the desired properties.

The magnetic layer described so far can be provided on the surface of the non-magnetic support, directly or indirectly through one or more other layers such as a non-magnetic layer which will be described later.

[Non-Magnetic Layer]

Next, a non-magnetic layer will be described.

The magnetic recording medium may have the magnetic layer directly on the surface of the non-magnetic support, or may have a non-magnetic layer containing non-magnetic powder and a binder between the non-magnetic support and the magnetic layer. The non-magnetic powder contained in the non-magnetic layer may be inorganic powder or organic powder. Furthermore, carbon black or the like can also be used. Examples of the inorganic powder include powder of a metal, a metal oxide, a metal carbonate, a metal sulfate, a metal nitride, a metal carbide, a metal sulfide, and the like. These non-magnetic powders can be obtained as commercially available products, or can be manufactured by known methods. For details of the non-magnetic powder, paragraphs “0036” to “0039” in JP2010-24113A can be referred to. The content (filling rate) of the non-magnetic powder in the non-magnetic layer is preferably within a range of 50% to 90% by mass, and more preferably within a range of 60% to 90% by mass.

For other details of the binder, the additives, and the like of the non-magnetic layer, known techniques relating to the non-magnetic layer can be applied. For example, regarding the type and content of the binder, the type and content of the additives, and the like, known techniques relating to the magnetic layer can also be applied.

In the present invention and the present specification, the non-magnetic layer also includes a substantially non-magnetic layer which contains non-magnetic powder with a small amount of ferromagnetic powder as an impurity or by intention, for example. Herein, the substantially non-magnetic layer refers to a layer having a remnant flux density of equal to or lower than 10 mT or a coercive force of equal to or lower than 7.96 kA/m (100 Oe) or having a remnant flux density of equal to or lower than 10 mT and a coercive force of equal to or lower than 7.96 kA/m (100 Oe). It is preferable that the non-magnetic layer does not have remnant flux density and coercive force.

[Non-Magnetic Support]

Next, a non-magnetic support (hereinafter, simply described as “support” as well) will be described. Examples of the non-magnetic support include known supports such as biaxially oriented polyethylene terephthalate, polyethylene naphthalate, polyamide, polyamide imide, and aromatic polyamide. Among these, polyethylene terephthalate, polyethylene naphthalate, and polyamide are preferable. These supports may be subjected to corona discharge, a plasma treatment, an easy adhesion treatment, a heat treatment, and the like in advance.

[Back Coating Layer]

The magnetic recording medium can have a back coating layer containing non-magnetic powder and a binder, on a surface side of the non-magnetic support opposite to a surface side provided with the magnetic layer. It is preferable that the back coating layer contains either or both of carbon black and inorganic powder. Regarding the binder contained in the back coating layer and various additives which can be optionally contained therein, known techniques relating to the formulation of the magnetic layer and/or the non-magnetic layer can be applied.

[Various Thicknesses]

The thickness of the non-magnetic support and each layer in the magnetic recording medium will be described below.

The thickness of the non-magnetic support is 3.0 to 80.0 μm for example, preferably 3.0 to 50.0 μm, and more preferably 3.0 to 10.0 μm.

The thickness of the magnetic layer can be optimized according to the saturation magnetization of the magnetic head to be used, the length of head gap, the band of recording signals, and the like. The thickness of the magnetic layer is generally 10 nm to 100 nm. From the viewpoint of high-density recording, the thickness of the magnetic layer is preferably 20 to 90 nm, and more preferably 30 to 70 nm. The magnetic layer may be constituted with at least one layer, and may be separated into two or more layers having different magnetic characteristics. Furthermore, the constitution relating to known multi-layered magnetic layers can be applied. In a case where the magnetic layer is separated into two or more layers, the thickness of the magnetic layer means the total thickness of the layers.

The thickness of the non-magnetic layer is equal to or greater than 50 nm for example, preferably equal to or greater than 70 nm, and more preferably equal to or greater than 100 nm. In contrast, the thickness of the non-magnetic layer is preferably equal to or less than 800 nm, and more preferably equal to or less than 500 nm.

The thickness of the back coating layer is preferably equal to or less than 0.9 μm, and more preferably 0.1 to 0.7 μm.

The thickness of each layer and the non-magnetic support of the magnetic recording medium can be measured by known film thickness measurement methods. For example, a cross-section of the magnetic recording medium in a thickness direction is exposed by known means such as ion beams or a microtome, and then the exposed cross-section is observed using a scanning electron microscope. By observing the cross-section, a thickness of one site in the thickness direction or an arithmetic mean of thicknesses of two or more randomly extracted sites, for example, two sites can be determined as various thicknesses. Furthermore, as the thickness of each layer, a design thickness calculated from the manufacturing condition may be used.

[Manufacturing Process]

<Preparation of Composition for Forming Each Layer>

The step of preparing a composition for forming the magnetic layer, the non-magnetic layer, or the back coating layer generally includes a kneading step, a dispersion step, and a mixing step that is performed if necessary before and after the aforementioned steps. Each of the aforementioned steps may be divided into two or more stages. The components used for preparing the composition for forming each layer may be added at the initial stage or in the middle of any of the above steps. As a solvent, it is possible to use one kind of solvent or two or more kinds of solvents generally used for manufacturing a coating-type magnetic recording medium. Regarding the solvent, paragraph “0153” in JP2011-216149A can be referred to. Furthermore, each of the components may be added in divided portions in two or more steps. For example, the binder may be added in divided portions in the kneading step, the dispersion step, and the mixing step performed after dispersion to adjust viscosity. In order to manufacture the aforementioned magnetic recording medium, the manufacturing techniques known in the related art can be used as various steps. In the kneading step, it is preferable to use an instrument having strong kneading force, such as an open kneader, a continuous kneader, a pressurized kneader, or an extruder. For details of the kneading treatment, JP1989-106338A (JP-H01-106338A) and JP1989-79274A (JP-H01-79274A) can be referred to. As a disperser, known ones can be used.

Regarding the dispersion treatment for the composition for forming a magnetic layer, as described above, it is preferable to inhibit the occurrence of chipping. In order to inhibit chipping, in the step of preparing the composition for forming a magnetic layer, it is preferable to perform the dispersion treatment for the ferromagnetic hexagonal ferrite powder in two stages, such that coarse aggregates of the ferromagnetic hexagonal ferrite powder are disintegrated in the first stage of the dispersion treatment and then the second stage of the dispersion treatment is performed in which the collision energy applied to the particles of the ferromagnetic hexagonal ferrite powder due to the collision with dispersion beads is smaller than in the first dispersion treatment. According to the dispersion treatment described above, it is possible to achieve both of the improvement of dispersibility of the ferromagnetic hexagonal ferrite powder and the inhibition of occurrence of chipping.

Examples of preferred aspects of the aforementioned two-stage dispersion treatment include a dispersion treatment including a first stage of obtaining a dispersion liquid by performing a dispersion treatment on the ferromagnetic hexagonal ferrite powder, the binder, and the solvent in the presence of first dispersion beads, and a second stage of performing a dispersion treatment on the dispersion liquid obtained by the first stage in the presence of second dispersion beads having a bead size and a density smaller than a bead size and a density of the first dispersion beads. Hereinafter, the dispersion treatment of the aforementioned preferred aspect will be further described.

In order to improve the dispersibility of the ferromagnetic hexagonal ferrite powder, it is preferable that the first and second stages described above are performed as a dispersion treatment preceding the mixing of the ferromagnetic hexagonal ferrite powder with other powder components. For example, in a case where the magnetic layer containing the abrasive and the aforementioned non-magnetic filler is formed, it is preferable to perform the aforementioned first and second stages as a dispersion treatment for a liquid (magnetic liquid) containing the ferromagnetic hexagonal ferrite powder, the binder, the solvent, and additives optionally added, before the abrasive and the non-magnetic filler are mixed with the liquid.

The bead size of the second dispersion beads is preferably equal to or less than 1/100 and more preferably equal to or less than 1/500 of the bead size of the first dispersion beads. Furthermore, the bead size of the second dispersion beads can be, for example, equal to or greater than 1/10,000 of the bead size of the first dispersion beads, but is not limited to this range. For example, the bead size of the second dispersion beads is preferably within a range of 80 to 1,000 nm. In contrast, the bead size of the first dispersion beads can be within a range of 0.2 to 1.0 mm, for example.

In the present invention and the present specification, the bead size is a value measured by the same method as used for measuring the aforementioned mean particle size of powder.

The second stage described above is preferably performed under the condition in which the second dispersion beads are present in an amount equal to or greater than 10 times the amount of the ferromagnetic hexagonal ferrite powder, and more preferably performed under the condition in which the second dispersion beads are present in an amount that is 10 to 30 times the amount of the ferromagnetic hexagonal ferrite powder, based on mass.

The amount of the first dispersion beads in the first stage is preferably within the above range.

The second dispersion beads are beads having a density smaller than that of the first dispersion beads. “Density” is obtained by dividing mass (unit: g) of the dispersion beads by volume (unit: cm³) thereof. The density is measured by the Archimedean method. The density of the second dispersion beads is preferably equal to or lower than 3.7 g/cm³, and more preferably equal to or lower than 3.5 g/cm³. The density of the second dispersion beads may be equal to or higher than 2.0 g/cm³ for example, and may be lower than 2.0 g/cm³. In view of density, examples of the second dispersion beads preferably include diamond beads, silicon carbide beads, silicon nitride beads, and the like. In view of density and hardness, examples of the second dispersion beads preferably include diamond beads.

The first dispersion beads are preferably dispersion beads having a density of higher than 3.7 g/cm³, more preferably dispersion beads having a density of equal to or higher than 3.8 g/cm³, and even more preferably dispersion beads having a density of equal to or higher than 4.0 g/cm³. The density of the first dispersion beads may be equal to or lower than 7.0 g/cm³ for example, and may be higher than 7.0 g/cm³. As the first dispersion beads, zirconia beads, alumina beads, or the like are preferably used, and zirconia beads are more preferably used.

The dispersion time is not particularly limited and may be set according to the type of the disperser used and the like.

<Coating Step>

The magnetic layer can be formed by directly coating a surface of the non-magnetic support with the composition for forming a magnetic layer or by performing multilayer coating by sequentially or simultaneously applying the composition for forming a magnetic layer. The back coating layer can be formed by coating a surface side of the non-magnetic support opposite to the surface side which has the magnetic layer (or will be provided with the magnetic layer) with the composition for forming a back coating layer. For details of coating for forming each layer, paragraph “0066” in JP2010-231843A can be referred to.

<Other Steps>

Regarding other various steps for manufacturing the magnetic recording medium, paragraphs “0067” to “0070” in JP2010-231843A can be referred to. It is preferable to perform an alignment treatment on the coating layer of the composition for forming a magnetic layer while the coating layer is staying wet (undried). For the alignment treatment, it is possible to apply various known techniques including those described in paragraph “0067” in JP2010-231843A without any limitation. As described above, from the viewpoint of controlling the XRD intensity ratio, it is preferable to perform a vertical alignment treatment as the alignment treatment. Regarding the alignment treatment, the above description can also be referred to.

As described above, according to an aspect of the present invention, there is provided a method for manufacturing the magnetic recording medium of an aspect of the present invention described above, which includes forming a magnetic layer through a step of preparing a composition for forming a magnetic layer, a step of forming a coating layer by coating the non-magnetic support with the prepared composition for forming a magnetic layer, directly or through at least another layer, and a step of performing a vertical alignment treatment on the coating layer, in which the step of preparing a composition for forming a magnetic layer includes a first stage of obtaining a dispersion liquid by performing a dispersion treatment on the ferromagnetic hexagonal ferrite powder, the binder, and a solvent in the presence of first dispersion beads and a second stage of performing a dispersion treatment on the dispersion liquid obtained by the first stage in the presence of second dispersion beads having a bead size and a density smaller than a bead size and a density of the first dispersion beads. Here, the aforementioned manufacturing method is an example of preferred manufacturing methods, and the aforementioned magnetic recording medium is not limited to the magnetic recording medium manufactured by the manufacturing method described above.

The aforementioned magnetic recording medium according to an aspect of the present invention can be a tape-like magnetic recording medium (magnetic tape), for example. Generally, the magnetic tape is distributed and used in a state of being accommodated in a magnetic tape cartridge. By mounting the magnetic tape cartridge on a drive (referred to as “magnetic tape device” as well) and running the magnetic tape in the drive such that a head contacts and slides on a surface of the magnetic tape (surface of a magnetic layer), signals are recorded on the magnetic tape and reproduced. In order to continuously or intermittently perform repeated reproduction of the signals recorded on the magnetic tape, the magnetic tape is caused to repeatedly run in the drive. According to an aspect of the present invention, it is possible to provide a magnetic tape in which the electromagnetic conversion characteristics thereof hardly deteriorate even if the head repeatedly slides on the surface of the magnetic layer while the tape is repeatedly running. Here, the magnetic recording medium according to an aspect of the present invention is not limited to the magnetic tape. The magnetic recording medium according to an aspect of the present invention is suitable as various magnetic recording media (a magnetic tape, a disc-like magnetic recording medium (magnetic disc), and the like) used in a sliding-type magnetic signal reproduction system and a magnetic signal reproduction device (drive). The sliding-type system and the drive refer to a system and a device respectively in which a head contacts and slides on a surface of a magnetic layer in a case where signals are recorded on a magnetic recording medium and/or reproduced.

EXAMPLES

Hereinafter, the present invention will be described based on examples, but the present invention is not limited to the aspects shown in the examples. In the following description, unless otherwise specified, “part” and “%” represent “part by mass” and “% by mass” respectively.

Example 1

The formulations of compositions for forming each layer will be shown below.

<Formulation of Composition for Forming Magnetic Layer>

(Magnetic Liquid)

Plate-like ferromagnetic hexagonal ferrite powder (M-type barium ferrite): 100.0 parts

(activation volume: 1,500 nm³)

Oleic acid: 2.0 parts

Vinyl chloride copolymer (MR-104 manufactured by ZEON CORPORATION): 10.0 parts

SO₃Na group-containing polyurethane resin: 4.0 parts

(weight-average molecular weight: 70,000, SO₃Na group: 0.07 meq/g)

Amine-based polymer (DISPERBYK-102 manufactured by BYK-Chemie GmbH): 6.0 parts

Methyl ethyl ketone: 150.0 parts

Cyclohexanone: 150.0 parts

(Abrasive liquid)

α-Alumina: 6.0 parts

(BET specific surface area: 19 m²/g, Mohs hardness: 9)

SO₃Na group-containing polyurethane resin: 0.6 parts

(weight-average molecular weight: 70,000, SO₃Na group: 0.1 meq/g)

2,3-Dihydroxynaphthalene: 0.6 parts

Cyclohexanone: 23.0 parts

(Non-magnetic filler liquid)

Colloidal silica: 2.0 parts

(mean particle size: 120 nm)

Methyl ethyl ketone: 8.0 parts

(Lubricant and curing agent liquid)

Stearic acid: 3.0 parts

Amide stearate: 0.3 parts

Butyl stearate: 6.0 parts

Methyl methyl ketone: 110.0 parts

Cyclohexanone: 110.0 parts

Polyisocyanate (CORONATE (registered trademark) L manufactured by Nippon Polyurethane Industry Co., Ltd.): 3.0 parts

<Formulation of Composition for Forming Non-Magnetic Layer>

Non-magnetic inorganic powder α iron oxide: 100.0 parts

(mean particle size: 10 nm, BET specific surface area: 75 m²/g)

Carbon black: 25.0 parts

(mean particle size: 20 nm)

SO₃Na group-containing polyurethane resin: 18.0 parts

(weight-average molecular weight: 70,000, content of SO₃Na group: 0.2 meq/g)

Stearic acid: 1.0 part

Cyclohexanone: 300.0 parts

Methyl ethyl ketone: 300.0 parts

<Formulation of Composition for Forming Back Coating Layer>

Non-magnetic inorganic powder α iron oxide: 80.0 parts

(mean particle size: 0.15 μm, BET specific surface area: 52 m²/g)

Carbon black: 20.0 parts

(mean particle size: 20 nm)

Vinyl chloride copolymer: 13.0 parts

Sulfonate group-containing polyurethane resin: 6.0 parts

Phenyl phosphonate: 3.0 parts

Cyclohexanone: 155.0 parts

Methyl ethyl ketone: 155.0 parts

Stearic acid: 3.0 parts

Butyl stearate: 3.0 parts

Polyisocyanate: 5.0 parts

Cyclohexanone: 200.0 parts

<Preparation of Composition for Forming Magnetic Layer>

The composition for forming a magnetic layer was prepared by the following method.

The aforementioned various components of a magnetic liquid were dispersed for 24 hours by a batch-type vertical sand mill by using zirconia beads (first dispersion beads, density: 6.0 g/cm³) having a bead size of 0.5 mmΦ (first stage) and then filtered using a filter having an average pore size of 0.5 μm, thereby preparing a dispersion liquid A. The amount of the used zirconia beads was 10 times the amount of the ferromagnetic hexagonal barium ferrite powder based on mass.

Then, the dispersion liquid A was dispersed for 1 hour by a batch-type vertical sand mill by using diamond beads (second dispersion beads, density: 3.5 g/cm³) having a bead size of 500 nmΦ (second stage), and the diamond beads were separated using a centrifuge, thereby preparing a dispersion liquid (dispersion liquid B). The following magnetic liquid is the dispersion liquid B obtained in this way.

The aforementioned various components of an abrasive liquid were mixed together and put into a horizontal beads mill disperser together with zirconia beads having a bead size of 0.3 mmΦ, and the volume thereof was adjusted such that bead volume/(volume of abrasive liquid)+bead volume equaled 80%. The mixture was subjected to a dispersion treatment by using the beads mill for 120 minutes, and the liquid formed after the treatment was taken out and subjected to ultrasonic dispersion and filtration treatment by using a flow-type ultrasonic dispersion and filtration device. In this way, an abrasive liquid was prepared.

The prepared magnetic liquid and abrasive liquid as well as the non-magnetic filler liquid, the lubricant, and the curing agent liquid described above were introduced into a dissolver stirrer, stirred for 30 minutes at a circumferential speed of 10 m/sec, and then treated in 3 passes with a flow-type ultrasonic disperser at a flow rate of 7.5 kg/min. Thereafter, the resultant was filtered through a filter having a pore size of 1 μm, thereby preparing a composition for forming a magnetic layer.

<Preparation of Composition for Forming Non-Magnetic Layer>

The aforementioned various components of a composition for forming a non-magnetic layer were dispersed by a batch-type vertical sand mill for 24 hours by using zirconia beads having a bead size of 0.1 mmΦ and then filtered using a filter having an average pore size of 0.5 μm, thereby preparing a composition for forming a non-magnetic layer.

<Preparation of Composition for Forming Back Coating Layer>

Among the aforementioned various components of a composition for forming a back coating layer, the components except for the lubricant (stearic acid and butyl stearate), polyisocyanate, and 200.0 parts of cyclohexanone were kneaded and diluted using an open kneader and then subjected to a dispersion treatment in 12 passes by a horizontal beads mill disperser by using zirconia beads having a bead size of 1 mmΦ by setting a bead filling rate to be 80% by volume, a circumferential speed of the rotor tip to be 10 m/sec, and a retention time in 1 pass to be 2 minutes. Then, other components described above were added thereto, followed by stirring with a dissolver. The obtained dispersion liquid was filtered using a filter having an average pore size of 1 μm, thereby preparing a composition for forming a back coating layer.

<Preparation of Magnetic Tape>

A surface of a support made of a polyethylene naphthalate having a thickness of 5.0 μm was coated with the composition for forming a non-magnetic layer prepared as above such that a thickness of 100 nm was obtained after drying, and then the composition was dried, thereby forming a non-magnetic layer. A surface of the formed non-magnetic layer was coated with the composition for forming a magnetic layer prepared as above such that a thickness of 70 nm was obtained after drying, and then the composition was dried, thereby forming a coating layer. While the coating layer of the composition for forming a magnetic layer is staying wet (undried), a vertical alignment treatment was performed on the coating layer such that a magnetic field having a magnetic field intensity of 0.15 T was applied in a direction perpendicular to a surface of the coating layer. Then, the coating layer was dried.

Thereafter, a surface of the support opposite to the surface on which the non-magnetic layer and the magnetic layer were formed was coated with the composition for forming a back coating layer prepared as above such that a thickness of 0.4 μM was obtained after drying, and then the composition was dried. By using a calendar constituted solely with metal rolls, a calendar treatment (surface smoothing treatment) was performed on the obtained tape at a speed of 100 m/min, a line pressure of 300 kg/cm (294 kN/m), and a calendar roll surface temperature of 100° C. Subsequently, the tape was subjected to a heat treatment for 36 hours in an environment with an atmospheric temperature of 70° C. After the heat treatment, the tape was slit in a width of ½ inches (0.0127 meters), thereby obtaining a magnetic tape.

The thickness of each of the above layers is a design thickness calculated from the manufacturing conditions.

The activation volume of the ferromagnetic hexagonal ferrite powder is a value measured and calculated using the powder that was in the same powder lot as the ferromagnetic hexagonal ferrite powder used in the composition for forming a magnetic layer. The activation volume was measured using a vibrating sample fluxmeter (manufactured by TOEI INDUSTRY, CO., LTD.) by setting a magnetic field sweep rate to be 3 minutes and 30 minutes for a coercive force Hc measurement portion, and calculated from the relational expression described above. The activation volume was measured in an environment with a temperature of 23° C.±1° C.

Example 2 to 4 and Comparative Example 1

Magnetic tapes were prepared in the same manner as in Example 1, except that the magnetic field intensity in the vertical alignment treatment was changed to the value shown in Table 1.

Example 5

A magnetic tape was prepared in the same manner as in Example 1, except that the dispersion treatment for the magnetic liquid was performed in two stages under the conditions shown in Table 1.

Comparative Example 2

A magnetic tape was prepared in the same manner as in Example 1, except that the vertical alignment treatment was not performed.

Comparative Example 3

A magnetic tape was prepared in the same manner as in Example 1, except that in the dispersion treatment for the magnetic liquid, the dispersion treatment using the first dispersion beads were continued for 1 more hour (accordingly, dispersion was performed for 25 hours in total), and the second stage and the vertical alignment treatment were not performed.

Comparative Example 4

A magnetic tape was prepared in the same manner as in Example 1, except that in the dispersion treatment for the magnetic liquid, the dispersion treatment using the first dispersion beads was continued for 1 more hour (accordingly, dispersion was performed for 25 hours in total), and the second stage was not performed.

Comparative Example 5

A magnetic tape was prepared in the same manner as in Example 3, except that in the dispersion treatment for the magnetic liquid, the dispersion treatment using the first dispersion beads was continued for 1 more hour (accordingly, dispersion was performed for 25 hours in total), and the second stage was not performed.

[Evaluation Method]

(1) XRD Intensity Ratio

From each of the magnetic tapes of examples and comparative examples, tape samples were cut.

By using a thin film X-ray diffractometer (SmartLab manufactured by Rigaku Corporation), X-rays were caused to enter a surface of the magnetic layer of the cut tape sample, and In-Plane XRD was performed by the method described above.

From the X-ray diffraction spectrum obtained by In-Plane XRD, a peak intensity Int (114) of a diffraction peak of (114) plane and a peak intensity Int (110) of a diffraction peak of (110) plane of the hexagonal ferrite crystal structure were determined, and the XRD intensity ratio (Int (110)/Int (114)) was calculated.

(2) Squareness Ratio (SQ) in Vertical Direction

For each of the magnetic tapes of examples and comparative examples, by using a vibrating sample fluxmeter (manufactured by TOEI INDUSTRY, CO., LTD.), a squareness ratio in a vertical direction was determined by the method described above.

(3) Deterioration of Electromagnetic Conversion Characteristics (Signal-to-Noise-Ratio; SNR)

The electromagnetic conversion characteristics of each of the magnetic tapes of examples and comparative examples were measured using a ½-inch (0.0127 meters) reel tester, to which a head was fixed, by the following method.

A head/tape relative speed was set to be 6 m/s. A Metal-In-Gap (MIG) head (gap length: 0.15 μm, track width: 1.0 μm) was used for recording, and as a recording current, a recording current optimal for each magnetic tape was set. As a reproducing head, a Giant-Magnetoresistive (GMR) head having an element thickness of 15 nm, a shield gap of 0.1 μm, and a lead width of 0.5 μm was used. Signals were recorded at a line recording density of 270 KFCI, and reproduced signals were measured using a spectrum analyzer manufactured by ShibaSoku Co., Ltd. A ratio between an output value of carrier signals and integrated noise in the entire bandwidth of the spectrum was taken as SNR. For measuring SNR, the signals of a portion of each magnetic tape in which signals were sufficiently stabilized after running were used.

Under the above conditions, each magnetic tape was caused to perform reciprocating running in 5,000 passes at 1,000 m/1 pass in an environment with a temperature of 32° C. and a relative humidity of 80%, and then SNR was measured. Then, a difference between SNR of the 1^(st) pass and SNR of the 5,000^(th) pass (SNR of the 5,000^(th) pass−SNR of the 1^(st) pass) was calculated. In a case where the difference is less than −2.0 dB, it is possible to make a determination that the magnetic tape exhibits excellent electromagnetic conversion characteristics required for a data backup tape.

The recording and reproduction of signals described above were performed by causing the head to slide on a surface of the magnetic layer of the magnetic tape.

For the magnetic tapes of Comparative Examples 1 to 5, the surface of the reproducing head was observed using an optical microscope (20× objective lens) after reciprocating running performed 5,000 passes. As a result of observation, it was confirmed that in Comparative Example 1, the head was partially scraped, that is, head scraping occurred. In Comparative Examples 2 to 4, it was confirmed that foreign substances resulting from the scraping of the surface of the magnetic layer were attached to the head, that is, the magnetic layer scraping occurred. In Comparative Example 5, it was confirmed that head scraping and magnetic layer scraping occurred.

The above results are shown in Table 1.

TABLE 1 Magnetic liquid dispersion treatment Magnetic liquid dispersion treatment First stage Second stage Dispersion beads Dispersion beads Formulated amount Formulated amount (mass of beads with (mass of beads with respect to mass of respect to mass of ferromagnetic ferromagnetic Type Bead size hexagonal ferrite powder) Time Type Bead size hexagonal ferrite powder) Time Example 1 Zirconia 0.5 mm 10 times greater 24 h Diamond 500 nm 10 times greater 1 h Example 2 Zirconia 0.5 mm 10 times greater 24 h Diamond 500 nm 10 times greater 1 h Example 3 Zirconia 0.5 mm 10 times greater 24 h Diamond 500 nm 10 times greater 1 h Example 4 Zirconia 0.5 mm 10 times greater 24 h Diamond 500 nm 10 times greater 1 h Example 5 Zirconia 0.5 mm 10 times greater 24 h Diamond 500 nm 20 times greater 1 h Comparative Zirconia 0.5 mm 10 times greater 24 h Diamond 500 nm 10 times greater 1 h Example 1 Comparative Zirconia 0.5 mm 10 times greater 24 h Diamond 500 nm 10 times greater 1 h Example 2 Comparative Zirconia 0. 5 mm 10 times greater 25 h N/A N/A N/A N/A Example 3 Comparative Zirconia 0.5 mm 10 times greater 25 h N/A N/A N/A N/A Example 4 Comparative Zirconia 0.5 mm 10 times greater 25 h N/A N/A N/A N/A Example 5 Vertical alignment Physical properties of magnetic tape Evaluation result treatment XRD intensity Squareness ratio Observation result Magnetic field ratio in vertical SNR after 5,000 passes intensity [Int (110)/Int (114)] direction deterioration of reciprocating running Example 1 0.15 T 0.5 0.70 −0.5 — Example 2 0.20 T 1.5 0.75 −0.3 — Example 3 0.30 T 2.3 0.80 −0.2 — Example 4 0.50 T 4.0 0.85 −0.6 — Example 5 0.15 T 0.7 0.83 −0.4 — Comparative 1.00 T 6.1 0.90 −2.1 Head scraping Example 1 Comparative N/A 0.3 0.66 −2.2 Magnetic layer scraping Example 2 Comparative N/A 0.2 0.55 −2.3 Magnetic layer scraping Example 3 Comparative 0.15 T 3.8 0.63 −3.0 Magnetic layer scraping Example 4 Comparative 0.30 T 5.0 0.75 −4.1 Head scraping and Example 5 magnetic layer scraping

From the results shown in Table 1, it was confirmed that the electromagnetic conversion characteristics of the magnetic tapes of examples hardly deteriorate even if reproduction is repeated by causing a head to slid on the surface of the magnetic layer.

INDUSTRIAL APPLICABILITY

The present invention is useful in the technical field of magnetic tapes for data storage such as data backup tapes. 

What is claimed is:
 1. A magnetic recording medium comprising: a non-magnetic support; and a magnetic layer which is provided on the support and contains ferromagnetic powder and a binder, wherein the ferromagnetic powder is ferromagnetic hexagonal ferrite powder, the magnetic layer contains an abrasive, an intensity ratio (Int (110)/Int (114)) of a peak intensity Int (110) of a diffraction peak of (110) plane of a crystal structure of the hexagonal ferrite, determined by performing X-ray diffraction analysis on the magnetic layer by using an In-Plane method, to a peak intensity Int (114) of a diffraction peak of (114) plane of the crystal structure is equal to or higher than 0.5 and equal to or lower than 4.0, and a squareness ratio in a vertical direction is equal to or higher than 0.65 and equal to or lower than 1.00.
 2. The magnetic recording medium according to claim 1, wherein the squareness ratio in a vertical direction is equal to or higher than 0.65 and equal to or lower than 0.90.
 3. The magnetic recording medium according to claim 1, wherein the intensity ratio Int (110)/Int (114) is equal to or higher than 1.0 and equal to or lower than 3.0.
 4. The magnetic recording medium according to claim 2, wherein the intensity ratio Int (110)/Int (114) is equal to or higher than 1.0 and equal to or lower than 3.0.
 5. The magnetic recording medium according to claim 1, further comprising: a non-magnetic layer containing non-magnetic powder and a binder between the non-magnetic support and the magnetic layer.
 6. A method for manufacturing the magnetic recording medium, wherein the magnetic recording medium is a magnetic recording medium comprising: a non-magnetic support; and a magnetic layer which is provided on the support and contains ferromagnetic powder and a binder, wherein the ferromagnetic powder is ferromagnetic hexagonal ferrite powder, the magnetic layer contains an abrasive, an intensity ratio (Int (110)/Int (114)) of a peak intensity Int (110) of a diffraction peak of (110) plane of a crystal structure of the hexagonal ferrite, determined by performing X-ray diffraction analysis on the magnetic layer by using an In-Plane method, to a peak intensity Int (114) of a diffraction peak of (114) plane of the crystal structure is equal to or higher than 0.5 and equal to or lower than 4.0, and a squareness ratio in a vertical direction is equal to or higher than 0.65 and equal to or lower than 1.00; and the method comprising: forming the magnetic layer through a step of preparing a composition for forming a magnetic layer, a step of forming a coating layer by coating the non-magnetic support with the prepared composition for forming a magnetic layer directly or through at least another layer, and a step of performing a vertical alignment treatment on the coating layer, wherein the step of preparing a composition for forming a magnetic layer includes a first stage of obtaining a dispersion liquid by performing a dispersion treatment on the ferromagnetic hexagonal ferrite powder, the binder, and a solvent in the presence of first dispersion beads and a second stage of performing a dispersion treatment on the dispersion liquid obtained by the first stage in the presence of second dispersion beads having a bead size and a density smaller than a bead size and a density of the first dispersion beads.
 7. The manufacturing method according to claim 6, wherein the second stage is performed in the presence of the second dispersion beads in an amount equal to or greater than 10 times the amount of the ferromagnetic hexagonal ferrite powder based on mass.
 8. The manufacturing method according to claim 6, wherein the bead size of the second dispersion beads is equal to or less than 1/100 of the bead size of the first dispersion beads.
 9. The manufacturing method according to claim 6, wherein the bead size of the second dispersion beads is within a range of 80 to 1,000 nm.
 10. The manufacturing method according to claim 6, wherein the density of the second dispersion beads is equal to or less than 3.7 g/cm³.
 11. The manufacturing method according to claim 6, wherein the second dispersion beads are diamond beads.
 12. The manufacturing method according to claim 6, wherein the squareness ratio in a vertical direction of the magnetic recording medium is equal to or higher than 0.65 and equal to or lower than 0.90.
 13. The manufacturing method according to claim 6, wherein the intensity ratio Int (110)/Int (114) of the magnetic recording medium is equal to or higher than 1.0 and equal to or lower than 3.0.
 14. The magnetic recording medium according to claim 12, wherein the intensity ratio Int (110)/Int (114) of the magnetic recording medium is equal to or higher than 1.0 and equal to or lower than 3.0.
 15. The manufacturing method according to claim 6, wherein the magnetic recording medium further comprises a non-magnetic layer containing non-magnetic powder and a binder between the non-magnetic support and the magnetic layer. 